Security is a growing problem in many environments, for example, airports. With the threat of terrorism on the rise, it is becoming increasingly important to be able to detect potentially hazardous materials, particularly those being carried through airport security gates. In practice, however, this is difficult because often only trace levels of the materials are present and many existing systems do not have the sensitivity to detect such low levels.
One known system for detecting hazardous materials in an airport environment uses Ion Mobility Spectroscopy (IMS). Particles are collected from a sample region, typically a security gate and transported via air currents or a sample line to a sample collection chamber. This is done until a sample of a pre-determined concentration is collected. Once the pre-concentrated sample is collected, the particles are then heated until they reach the gas phase and are subsequently ionized so that their mass can be determined via a measure of the time of flight between two charged plates. The molecular mass provides an indication of the identity of particles collected.
A problem with IMS is that many different molecules are found to have similar masses making this susceptible to both false positive and false negative identification. A further problem is that the sample area over which the particles are collected is relatively limited, leaving the possibility of particles of interest not being captured. In addition, the sample has to be pre-concentrated, heated and ionized which means that the measurement response time is typically of the order of tens of seconds. This is too slow for many security applications.